Reactions of ferrocenylboranes with 2,5-bis(pyridyl)pyrazine and quaterpyridine: charge-transfer complexes and redox-active macrocycles

Abstract

Reactions of FcB(Me)Br [1; Fc = (C₅H₅)Fe(C₅H₄)] and 1,1′-fc[B(Me)Br]₂ [2; fc = (C₅H₄)₂Fe] with 2,5-bis(pyridyl)pyrazine (bppz) and 2,2′:4′,4″:2″,2‴-quaterpyridine (qpy) are reported. The pyrazine derivative bppz forms stable boronium cations with 1 and 2 to give the complexes [FcB(Me)bppz]Br, [1C]Br, and {1,1′-fc[B(Me)bppz]₂}Br₂, [2C₂]Br₂. The 4,4′-bipyridine derivative qpy can be used to link two ferrocenylborane moieties together, which gives access to the open-chain dinuclear aggregate [FcB(Me)qpyB(Me)Fc]Br₂, [1D1]Br₂, and to the macrocyclic molecule [2D]₂Br₄. The bppz adducts possess an intense green colour, and the qpy complexes are deeply blue coloured, which is indicative of charge-transfer interactions between the electron-rich ferrocene moieties and their electron-poor aromatic substituents. The hexafluorophosphate salts of all compounds undergo a reversible ferrocene oxidation and several consecutive reduction processes, which are centred at the cationic sidechains.